The present invention is directed to a fiber optic tunable filter using a Fabry-Perot resonator and more specifically to a filter having a flattopped passband.
In recent years, much attention has been paid to the development of a tunable optical filter based on a Fabry-Perot resonator. Two types of Fabry-Perot filters have been investigated. One type is a non-tunable filter where the mirror spacing is fixed and is equal to the layer thickness (that is the mirror layers over the dielectric) deposited during fabrication. In a tunable filter one of the mirrors is movable, resulting in adjustable mirror spacing. The foregoing, however, do not simultaneously satisfy the two important requirements of a wavelength-division multiplexing (WDM) filter; that is, wavelength tunability and a flattopped passband.
In a regular Fabry-Perot resonator or etalon (where the etalon is a slab of dielectric material of a certain index of refraction and thickness) the bandwidth of each transmission peak can be narrow and only one wavelength is transmitted with maximum transmission. If a regular Fabry-Perot etalon is made into a tunable filter, this is done by selecting the index of refraction of the medium in the cavity to select a specific resonant wavelength. The wavelength in resonance with the optical length of the cavity is transmitted, whereas the other wavelengths are reflected. Thus, a tunable Fabry-Perot filter where, for example, only one reflective mirror is used and this mirror is vibrated at a frequency of one to three kHz might produce with a complex frequency input a output such as shown in FIG. 1 where there are two wavelength peaks at different amplitudes and other wavelengths much lower in amplitude and overlapping. As discussed above, this is eminently unsuitable for WDM.
It is a general object of the present invention to provide improved fiber optic tunable filter.
In accordance with the above object there is provided a fiber optic tunable filter using a Fabry-Perot resonator comprising a pair of opposed interference filters each having three or more layers and spaced apart around a Fabry-Perot cavity a distance substantially equal to an integral number of wavelengths of the cavity""s resonant frequency. Input and output optical fibers are coupled to the cavity via the interference filters respectively, the input fiber transmitting an incident light beam having a plurality of nondistinct wavelengths near the resonant wavelength. Means are provided for vibrating one of the interference filters in the direction of the light beam whereby a plurality of distinct wavelengths of substantially equal amplitude is produced on the output fiber.